1. Field of the Invention
This invention relates to a method of making abrasive compositions, and more particularly, it relates to a method of making abrasive compositions comprised of aqueous suspensions of water-insoluble abrasive polishing agents and humectant, with abrasive particle comminution effected in post-reactor processing under wet conditions, and the products thereof.
2. Description of the Related Art
An abrasive substance has been included in conventional dentifrice compositions in order to remove various deposits, including pellicle film, from the surface of teeth. Pellicle film is tightly adherent and often contains brown or yellow pigments which impart an unsightly appearance to the teeth. While cleaning is important, the abrasive should not be so aggressive so as to damage the teeth. Ideally, an effective dentifrice abrasive material maximizes pellicle film removal while causing minimal abrasion and damage to the hard tooth tissues. Consequently, among other things, the performance of the dentifrice is highly sensitive to the abrasive polishing agent ingredient. Conventionally, the abrasive polishing material has been introduced in flowable dry powder form to dentifrice compositions, or via re-dispersions of flowable dry powder forms of the polishing agent prepared before or at the time of formulating the dentifrice.
A number of water insoluble, abrasive polishing agents have been used or described for dentifrice compositions. These abrasive polishing agents include natural and synthetic abrasive particulate materials. The generally known synthetic abrasive polishing agents include amorphous precipitated silicas, silica gels, dicalcium phosphate and its dihydrate forms, calcium pyrophosphate and precipitated calcium carbonate (PCC). Other abrasive polishing agents for dentifrices have included chalk, magnesium carbonate, zirconium silicate, potassium metaphosphate, magnesium orthophosphate, tricalcium phosphate, and the like.
Synthetically-produced precipitated silicas, in particular, have been used as abrasive components in dentifrice formulations due to their cleaning ability, relative safeness, and compatibility with typical dentifrice ingredients, such as humectants, thickening agents, flavoring agents, anti-caries agents, and so forth. As known, synthetic precipitated silicas generally are produced by the de-stabilization and precipitation of amorphous silica from soluble alkaline silicate by the addition of a mineral acid and/or acid gases under conditions in which primary particles initially formed tend to associate with each other to form a plurality of agglomerates (i.e., discrete clusters of primary particles), but without aggregation into a three-dimensional gel structure. The resulting precipitate is separated from the aqueous fraction of the reaction mixture by filtering, washing, and drying procedures, and then the dried product is mechanically comminuted in order to provide a suitable particle size.
The silica drying procedures are conventionally accomplished using spray drying, nozzle drying (e.g., tower or fountain), flash drying, rotary wheel drying, oven/fluid bed drying, and the like, which often require considerable expenditures for equipment and operating costs. A similar issue is associated with other synthetically derived polishing agents, such as silica gel and PCC.
Additionally, conventional abrasive polishing agents intended for dentifrices have required comminution in order to reduce the particle size of the dried precipitated silica product down to a size that does not feel gritty in the mouth of a dentifrice user, while, on the other hand, not being so small as to lack sufficient polishing action. That is, in conventional practice, the median particle size of the silica in the reactor formed by acidulation of a metal silicate is too large for dentifrice applications and the like. To comminute dry silica particulates, grinding and milling equipment have been used, such as a hammer or a pendulum mill used in one or multiple passes, and fine grinding has been performed, for example, by fluid energy or air-jet mill. These additional dry comminution operations entail added cost and process time.
U.S. Pat. No. 3,506,757 to Salzmann describes liquid dentifrices comprising particulate abrasive materials, stably suspended in an aqueous liquid vehicle with the aid of a polysaccharide gum as suspending agent. Similarly, PCT published application no. WO 97/46485 describes silica having a median particle size generally around 12 to 30 xcexcm provided in the form of a suspension, which can be stabilized using a hydrocolloid, particularly naming polysaccharides such as xanthan gum, guar gum, and water-soluble cellulose ethers. U.S. Pat. No. 5,310,543 describes liquid dentifrices containing particulate siliceous abrasive cleaning agents stably suspended in a liquid medium with the aid of a polysaccharide gum and using a liquid medium specified as being substantially free from polyol-type humectants in order to obtain satisfactory rheological properties.
Among other things, rheologically stable liquid abrasive compositions containing appropriately sized abrasive particles would be desirable that could be prepared without the need for costly drying and dry milling/comminuting post-treatments.
The above and other objectives, advantages and benefits are achieved by the present invention directed to a method of making abrasive compositions comprised of aqueous suspensions of water-insoluble abrasive polishing agents in combination with humectant, with abrasive particle comminution effected during post-reactor processing under wet conditions.
Generally, the invention is directed to a method for preparing an abrasive composition, comprising the steps of introducing, into a reactor container, reaction mixture contents comprising alkali silicate and acid with inter-mixing thereof to form precipitated silica; partially dewatering of the reaction mixture after forming the precipitated silica with filtration to provide a wet or press cake; wet grinding the precipitated silica to achieve a desired median particle size in the suspended abrasive particles without any intervening step(s) of drying or dry milling being performed on the precipitated silica; and dispersing the wet-ground silica suspension with humectant either before or after the wet grinding step. Preferably, the wet milled abrasive composition prepared by this invention will have precipitated silica particles having a median particle size of about 1 micron to about 30 micron, more preferably about 3 micron to about 15 micron.
As several embodiments of the invention which meeting these objectives, the wet grinding step can be performed directly on the wet cake obtained from vacuum filtration, or, alternatively, on the press cake obtained from pressure filtration, with an intervening fluidizing step being performed on the press cake, and without any intervening drying or dehydration step being used which would reduce the water content of the cake below 5 wt % and more preferably not below about 20 wt % before performing the aforementioned wet grinding procedure on the cake.
In yet another embodiment, the invention concerns a method for preparing an abrasive composition in combination with humectant, comprising the steps of introducing, into a reactor container, reaction mixture contents comprising alkali silicate and acid with inter-mixing thereof to form precipitated silica; followed by partial dewatering of the reaction mixture after forming the precipitated silica by pressure filtration to provide a press cake and washing the press cake; then, fluidizing the precipitated-silica in the press cake by combining humectant with the precipitated silica to provide a suspension of abrasive particles containing humectant; and wet grinding the suspension of abrasive particles containing humectant, to thereby achieve the desired median particle size in the suspended abrasive particles without the need for drying and/or dry comminution processing. In this alternate embodiment, the wet grinding is performed on a fluidized version of the press cake by using humectant, instead of directly on the press cake per se.
Again, prior to the wet grinding procedure, the silica suspension, whether in press or wet cake form from pressure or vacuum filtration, or in a fluidized form after pressure filtration downstream from the silica reactor, is never dehydrated below 5 wt % water, and preferably never below about 20 wt % water, and preferably these hydraulic conditions are maintained in the abrasive composition being processed continuously from the time of silica synthesis through wet grinding of the abrasive composition.
Preferably, the wet grinding used in the above-mentioned various embodiments of the invention is accomplished by wet media milling, either as a single stage or multi-stage procedure.
Using this invention, the water-insoluble abrasive particulate product of a silica acidulation reaction can be modified to have the requisite particle size suited for oral cleaning compositions to eliminate the need for drying and comminuting dry silica solids. While not desiring to be bound to any particular theory at this time, it is postulated that the rheological properties of the abrasive compositions prepared by this invention are superior, at least in part, due the avoidance of agglomeration of particles otherwise incurred by the silica particles during conventional silica particle drying procedures. xe2x80x9cDryingxe2x80x9d of the silica particles, for purposes herein, means silica particles have been dehydrated to an extent that a generally dry flowable powder results as the water content is reduced below about 10 wt %. Thus, xe2x80x9cdriedxe2x80x9d or xe2x80x9cdryxe2x80x9d abrasive particles have been subjected to such drying, as defined above. By contrast, the abrasive composition products made by the inventive method contain undried silica particles that retain or essentially retain the original structure and chemistry of the reactor silica.
Moreover, the method of the invention can be practiced more economically because the time as well as the equipment and operating costs otherwise required for particle drying procedures are eliminated. Additionally, the resulting abrasive compositions made by the inventive method are rheologically stable, settling-resistant, and re-agglomeration resistant, even during and after transport and/or storage before end-use. The abrasive compositions made by the inventive method are ready-to-use additives for the preparation of oral cleaning compositions, such as dentifrices, toothpastes, and the like, particularly suited as a raw material in a continuous toothpaste making process.